Currently, in order to determine the position of mobile elements of the type described above it is known to use a device comprising a source of a field of forces, normally a magnetic or electrical dipole, which is located on one of the two elements, preferably on the mobile element, and a field sensor, which is set in a given point of the other element, preferably the fixed one, and has the function of measuring, instant by instant, the field of forces in that point in order to produce, on the basis of said measurement, a response identifying the relative position of the source with respect to the sensor itself.
A device of this type, even though commonly used for its characteristics of simplicity and economy, presents, however, some considerable drawbacks, which derive principally from the relatively high approximation of the response supplied, which renders the device itself ineffective, if not even unserviceable, in the cases where a high precision of measurement is required.
Said approximation is intrinsic to the known type of device described above and derives from the fact that, since the sensors commonly used are sensitive both to the direction and to the modulus of the field vector, the sensor produces, for a given position of the source, a response that is a function both of the intensity and of the direction of the field generated by the source in that position.
It follows that the sensor supplies the same response for all the intensity-direction pairs that produce the same output in the sensor, even though each pair corresponds to a respective different position of the source.
In graphic terms, if the response of the sensor is represented in a reference system centred in the source, it is noted that, for a given value of the field corresponding to a given position of the source, the output of the sensor does not identify a precise point, corresponding to the point in which the source is located, but a set of points forming a complex surface, each corresponding to a position in which the source could be located. In practice, the complex surface referred to above is an iso-output surface formed by the locus of the points that, if occupied by the source, determine issuing, by the sensor, of the same output.